This invention relates to electronically addressed displays which are capable of displaying alphanumeric, graphic and pictorial information translated from an electrical signal supplied to the display. Such displays are widely used with computer terminals, word processors, and other information handling equipment where it is desired to view electronically stored or transmitted information of an alphanumeric, graphic or a pictorial nature. More specifically, this invention relates to an electronically addressed electro optical display which is compact, relatively thin, portable and yet having a display size, resolution and contrast permitting the display of page sized, typewritten, alphanumeric, graphic and pictorial information.
The electronic display most widely used in conjunction with information handling equipment is the cathode ray tube. While the cathode ray tube possesses a number of well known advantageous features, it also possesses certain disadvantageous features which are more or less inherent in its basic design: for example, a cathode ray tube requires relatively high voltages in its operation, is susceptible to electromagnetic interference and is relatively bulky and heavy, thus limiting its portability. In addition, the cathode ray tube cannot be made economically in relatively large sizes, such as with a display area of 4 to 10 square feet. Finally, certain health hazards can be presented in the use of cathode ray displays. The Swedish Occupational Health and Safety Board has recommended that cathode ray tube terminal operators view them for only short periods to avoid eye strain and similar problems. X-ray radiation and implosion are other potential health hazards presented in connection with the use of cathode ray tube displays.
A variety of other display devices have been suggested. One class of such devices are the so called passive displays which operate by selectively modulating, in response to an electrical signal, the transmission, reflection, or scattering of light to create an image. Included in this class are, among others, liquid crystal devices, electrophoretic devices, and electrochromic devices. The other major class of electronically addressable displays are the so called active element displays which emit visible light in response to an electrical impulse to create an image. Included in this class are various types of gas discharge panels, electroluminescent panels, and the like. Limitations on the desirability of some types of active element displays have been their expense, including difficulty and expense of construction and relative bulkiness of units, including associated addressing circuitry, when made in sizes on the order of 9 inches by 12 inches or larger, which are those sizes commonly employed as information handling equipment terminal displays. Further limitations include monochromatic operation in colors unpleasing to and causing strain to the eye and, in some cases, high voltage operation and relatively poor resolution. The passive displays, usually in matrix form, have the advantage of relative thinness at least in small sizes and particularly in the case of liquid crystal devices, the advantage of low voltage, low power operation. In addition, such displays, particularly liquid crystals, have the capability of operation under high light level conditions. A major limitation on the use of passive displays has been an inability to scale up the size of such displays to current CRT terminal display sizes on the order of 9 inches by 12 inches or larger. A major reason for this limitation has been the expense and difficulty of fabricating 9 inch by 12 inch size or larger thin displays with 900 by 1200 line dot matrices with the associated electronic matrix address circuits required for such displays. In addition, enlarging the matrix shortens the life of certain of the passive displays, particularly when producing a display having a resolution capable of displaying typewritten size material. A possible solution to this problem would be the creation of a modular construction in which an array of passive display elements, for example, liquid crystals, would be employed. The difficulty with this solution by itself is that the separation lines between image bearing portions of the display element modules would, of necessity, be quite wide in order to provide space for the seal if necessary and the electronic addressing circuitry access. Such lines would substantially disfigure the image displayed and would diminish the amount and accuracy of information which could be displayed in such a display construction.